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Overcurrent supervision of the DMU
The ground fault measuring units are supervised by IØs detectors.
The phase to phase fault measuring units phases are supervised by IØØs
detectors.
This way, although the difference of the operation and polarization quantity
arguments, for example, of the AG unit, is between +90° and -90°, if the phase A current A
does not exceed the set current threshold, the AG distance measuring unit will not be
activated.
IØs:
Phase to ground overcurrent detectors: the threshold is adjustable (typically
15% of In).
IØØs: Phase to phase overcurrent detectors; the threshold is adjustable (typically
15% of In).
Operating Security.
The MLP contains an algorithm, based on the operating quantity, which verifies the
system trip decision. The main objective of the additional function is to avoid the transients
that are produced by resistive voltage transformers which tend to produce overreaches.
This algorithm generally avoids overreaches for fault that are close to the limit of the first
zone. The programmed delay for this algorithm, which is exclusively for faults close to the
limit of the first zone, is 15 ms).
4.4 PROTECTION SCHEMES
The MLP includes three protection schemes:
0 - Scaled distance:
Three line distance protection Zones for faults between ground-phase and
phases.
1 - Zone 1 extension.
2 - Zone 2 acceleration. (MLP3000)
The last two schemes cannot be selected simultaneously and the first one is active
when selecting one of the other two.
The protection logic can be seen in the block diagram of figure 4. The trip decision
logic can be seen in figures 5.1, 5.2, 5.3 and 5.4. The protection schemes for zone 1
extension and zone 2 acceleration can be seen in figures 6.1 and 6.2.
Scaled distance
To better understand the scaled distance scheme, we will discuss how the
protection operates under five different situations:
1 - Fault inside the zone 1 reach.
2 - Fault inside the zone 2 reach
GEK-98846A
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